You may be in a situation where you have an existing Windows virtual machine that has just a single CPU assigned but now needs more processing power. This is generally a simple case of powering down the vm and altering the properties to the number of vCPUs required, before powering back up (unless you’re lucky enough to be using vSphere with a supported Windows release that allows hot CPU add). The HAL is generally detected and updated on system restart.
However, if the HAL isn’t updated you may still only be able to see one CPU in the guest vm. You can force the system to re-evaluate the HAL by opening msconfig.exe > Boot > Advanced Options and then check the ‘Detect HAL’ option. Also available from this menu is the option to select the relevant number of CPUs manually.